The preferred representation of strings on the stack is addr
u-count, where addr is the address of the first character and
u-count is the number of characters in the string.

v4 5 type

You get a string constant with

s" hello, world" .s
type

Make sure you have a space between s" and the string; s"
is a normal Forth word and must be delimited with white space (try what
happens when you remove the space).

However, this interpretive use of s" is quite restricted: the
string exists only until the next call of s" (some Forth systems
keep more than one of these strings, but usually they still have a
limited lifetime).

s" hello," s" world" .s
type
type

You can also use s" in a definition, and the resulting
strings then live forever (well, for as long as the definition):